1. Field of the Invention
The present invention relates to a power supply control device and a method of controlling the same. More particularly, the invention relates to a power supply control device with a semiconductor switch for controlling the supply of electric power from a power supply to a load by its switching control in response to a control signal, and a power supply control method for controlling the same.
2. Related Art
A power supply control device with a semiconductor switch as shown in FIG. 19 is known. This conventional power supply control device supplies electric power from a battery selectively to each load in a motor vehicle, and controls the power supply to the load.
As seen from the figure, the power supply control device is arranged such that a shunt resistor RS and the drain (D)--source (S) path of a thermal FET QF are connected in series in a path for applying an output voltage VB of a power source 101 to a load 102, such as head lamps and drive motors for power windows. The power supply control device includes a driver 901 for detecting a current flowing through the shunt resistor RS and controlling the drive of the thermal FET QF by a hardware circuit, an A/D converter 902 for performing an on/off control of a drive signal to the thermal FET QF in accordance with a current value monitored by the driver 901, and a microcomputer (CPU) 903.
The thermal FET QF, containing a temperature sensor (not shown) therein, has such an overheat cut-off function that when a temperature of the thermal FET QF rises to a predetermined temperature or higher, a gate cut-off circuit contained therein forcibly turns off the thermal FET QF. In the figure, RG represents a resistor RG, and ZD1 is a Zener diode which keeps a voltage across the gate (G)-source (S) path at 12V, and when an overvoltage will reach the gate G, provides a by path for the overvoltage.
The conventional power supply control device has also a protection function against an overcurrent flowing through the load 102 or the drain-source path of the thermal FET QF. The power supply control device includes a driver 901, differential amplifiers 911 and 913 as current monitoring circuits, another differential amplifier 912 as a current restriction circuit, a charge pump circuit 915, and a drive circuit 914 for driving the gate G of the thermal FET QF in accordance with an on/off control signal from the microcomputer 903 and an overcurrent judging result signal from the current restriction circuit.
When it is judged through the differential amplifier 912 that the current exceeds a judging value (upper limit) of current, by a voltage drop across the shunt resistor RS, viz., an overcurrent is detected, the drive circuit 914 turns off the thermal FET QF, and when the current decreases below a judging value (lower limit), it turns on the thermal FET QF.
The microcomputer 903 always monitors the current by use of the current monitoring circuit (differential amplifiers 911 and 913). If an abnormal current flows which is in excess of a normal current in value, it interrupts the drive signal to the thermal FET QF to turn off the thermal FET QF. Before the microcomputer 903 outputs a drive signal for an off control, when a temperature of the thermal FET QF exceeds a predetermined value of temperature, the thermal FET QF is turned off by the overheat cut-off function.
In the conventional power supply control device, the shunt resistor RS connected in series to the power supplying path is required for the current detection. The on-resistance of the recent thermal FET QF reduces, so that the load current is large. For this reason, a heat loss by the shunt resistor amounts to a quantity not negligible.
The overheat cut-off function and the overcurrent restriction circuit effectively function when a large current flows because of a dead short in the load 102 or wirings. When an imperfect short-circuit, such as a layer short-circuit, having a certain amount of short resistance occurs, and a small short-circuit current flows, the overheat cut-off function and the overcurrent restriction circuit fail to function. In this case, only one optional way permitted is that the microcomputer 903 detects an abnormal current by the current monitoring circuit and turns off the thermal FET QF. However, the control by the microcomputer has a disadvantage of slow response to the abnormal current.
The use of the shunt resistor RS, the microcomputer 903 and the like indispensably requires a large mounting space. Further, those components are relatively expensive, so that cost to manufacture the power supply control device is high.